Light sensitive layer for electrophotography



United States Patent US. Cl. 961.8 6 Claims ABSTRACT OF THE DISCLOSURE A light sensitive photographic layer containing photoconductive zinc oxide powder, an electrically insulating fihn-forming material and a water insoluble cupric compound. A process :for forming the light sensitive layer comprises mixing photoconductive zinc oxide powder and a finely divided cupric compound with a solution of an organic film-forming material, and then coating the resulting mixture onto a suitable support.

This invention relates to a novel light sensitive layer for electrophotography using zinc oxide as the photoconductive material.

The characteristics of a light sensitive layer for electrophotography consisting of photoconductive zinc oxide and an electrically insulating film forming material are remarkably influenced by the change of the film forming material which will be designated as a binding agent hereinafter.

Important points required for the electrophotographic characteristics of such a layer are as follows:

(I) High initial surface potential (a potential just after changing -by a corona discharge in a dark place,

(H) Suitably slow dark attenuation,

(HI) Rapid light attenuation and low residual potential, and

(IV) No formation of fog during developing.

We have tested a number of binder compositions and found that many of them can be put to practical use. Furthermore, it is found that the degree of influence on the characteristic (I) by radiating light prior to charging is remarkably changed by the nature of the binding agent composition in particular. Therefore, the following condition (V) must be added:

(V) The effect of pre-exposure to light is hardly suffered or can be eliminated as fast as possible.

A state where the influence by the pre-exposure to light remains is called light-adapted, while a state where such an influence is eliminated by storing in a dark place for a long time is called dark-adapted. When a layer in the light-adapted state (fatigued state) 18 subjected to corona discharge, the potential is low and the dark attenuation velocity is high. In order to bring this to the dark-adapted state, it is effective to allow it to stand in a dark place for a long time, to heat in a dark place or to subject to a negative corona discharge.

As the corona discharge itself causes a layer to recover from the light-adapted state to the dark-adapted state, it is better, for the correct judgment of the degree of fatigue, to observe a curve of charging time-surface potential rather than to measure an initial surface potential. In a dark-adapted layer, left standing in a dark place for a long time, the rising gradient of the charge curve is high-pitched and the saturation potential value 3,494,766 Patented Feb. 10, 1970 ice is high, while in a light-adapted layer, the gradient is gentle and the saturation potential value is low.

The recovering velocity of fatigue varies to a great extent with a binding agent combined with zinc oxide. We have found as a result of studies that silicone resin is the most excellent in this point but many other resins such as alkyd, epoxy, epoxy ester, copolymer of vinyl chloride and acetate, acrylic acid ester, methacrylic acid ester, polyvinyl butyral, copolymer of styrene and butadiene, sulfonamide and polyketone, and their miscible combinations compare unfavorably with silicone 'resin, that is, the recovering velocity is much lower. With respect to thermosetting or setting resins, the recovering velocity is lowered after hardening rather than therebefore. We have invented and filed an application about a new method utilizing this phenomenon (Japanese patent application No. 39,489/63). In this invention, effective additives capable of raising the recovery velocity remarkably for any binding agent composition are discovered.

We have now found in accordance with this invention that the fatigue recovering character of electrophotographic elements can be greatly improved by the combination with zinc oxide of a finely divided water-insoluble compound containing cupric ion. This method is more convenient and effective as compared with methods of treating zinc oxide with aqueous solution of inorganic salt.

Illustrative of the effective cupric compound are cupric oxide, cupric sulfide, cupric selenide, cupric hydroxide, cupric carbonate, basic cupric carbonate, cupric phosphate, cupric arsenite, cupric formate, cupric oxalate, cupric oleate, cupric stearate and cupric salts of organic acids.

In a light sensitive layer of zinc oxide in the lightadapted state, the adsorption of oxygen to the surface of zinc oxide is hindered by some factor. That a binding agent retards the recovery is likley due to the mechanism wherein a functional group in the binding agent is absorbed on the surface of zinc oxide in place of oxygen or ties up available oxygen.

The cupric compound promotes the absorption progress by some mechanism, it is thought. Although it is not clear whether a part of cupric ions is partially doped in the surface of zinc oxide or not, the doping may occur partially considering that a remarkable effect is brought by mixing zinc oxide and a binding agent with cupric ion in an amount of only 0.002% by weight of the zinc oxide.

The amount of cupric salt to give a clear effect may be more than 0.002% of cupric ion by weight of the zinc oxide. The resistance to exposure to light having a high intensity of illumination is produced with the increase of the amount added to zinc oxide. Even if a large amount of such water-insoluble copper compound is added, other electrophotographic characteristics are scarcely influenced. Approximately up to 20% by weight of cupric ion may be added to zinc oxide.

As it has already been known that the fatigue phenomenon is dependent upon the surface layer of a light sensitive layer, the light sensitive layer of our invention may be adapted only to the surface layer, the lower layer being free of the copper compound. In the case of a light sensitive layer having a thickness of about 20 microns, if only the surface layer of 2-5 microns or more is composed of a layer having a high fatigue restoration, the character of this light sensitive layer is represented by that of the surface layer.

Where the binding agent is a setting composition, the fatigue restoration is retarded by the setting in the conventional manner, but the presence of cupric salt improves such defect somewhat.

Among the cupric compounds, cupric stearate has such specific advantage besides the above-mentioned onesthat when the stearate is added in the coating liquid for the electrophotographic sensitive layer containing zinc ox 1de particles, it acts remarkably to prevent the precip tation of the zinc oxide particles as well as easily redisperse precipitated Zinc oxide particles in the coating llqllld, which improves the stability and the preservabihty of the coating liquid. That is, a large amount of cupric stearate makes the fluidity of the liquid thixotropic, resulting 1n the difliculty in some coating methods but preventing, if properly used, the sedimentation of inorganic pigments such as zinc oxide, whereby the storage property of the coating liquid can be improved. Therefore, the use of the stearate together with another copper salt is preferable to bring two or more improvements simultaneously. Of course, combinations of two or more other cup-ric salts also possible.

Moreover, as the copper compound has such an effect as to improve the dark attenuation character, it will be possible to add a material worsening the dark attenuation character, for example, a sensitizer, the addition thereof being undesired because of this.

This is shown in Examples IV and V, wherein the presence of the copper compound overcomes the disadvantage arising from the use of aluminum stearate alone, which acts to increase the dispersiveness of a pigment but to worsen the dark attenuation character. Similar effect' can be found in the joint use with a sensitizing dye.

' In addition, the electrophotoconductive layer of the present invention can be used not only for general printings in office but also for marking.

The invention will be more fully understood in the light of the following examples:

EXAMPLE I Composition A Part by weight Photoconductive zinc oxide powder 100 Soya-bean oil type alkyd resin varnish (soya-bean oil modifying, oil length 40%) (non-volatile 50%) 30 Low polymerization degree silicone resin varnish (non-volatile 60%) 30 Cobalt naphthenate 1.0 Methyl ethyl ketone peroxide 0.5 Toluene 20 The foregoing composition was charged to a ball mill of porcelain and mixed adequately.

Composition B Initial surface potential (volt) 450-500 450-500 Half time for dark attenuation (min.) 6-8 8-9 Relative sensitivity for xenon light source 100 70 Initial potential when charged after pre-exposure to light of 5,000 lux from a fluorescent lamp for min. and standing in a dark place for 2 min. (volt) 70-80 250-300 In B, the sensitivity 'is somewhat lowered but this does not harm to the practical use. The restoration from the fatigue is faster than in A.

EXAMPLE II Silicone resin varnish (non-volatile 60%) 18 Styrene-modified alkyl resin varnish (non-volatile 50% 32 Toluene 10 Methyl ethyl ketone 10 Composition D Composition C was mixed with the following components:

Part by weight Cupric stearate 0.2 Cupric oxalate 0.2

The characteristics are as follows:

Initial surface potential (volt) 450-500 450-500 Halt time for dark attenuation (min.) About 5 About 7 Relative sensitivity for xenon light source 100 Initial potential after pro-exposure to light of 500 lux from a fluorescent lamp for 5 min. and standing in a dark place for 5 min. (volt) -150 400-460 EXAMPLE III In a similar manner, the following two compositions were applied to an aluminum plate to give a thickness of 20 microns (dry base):

Lead chromate, 15 part by weight.

Composition B Part by weight Photoconductive zinc oxide 100 Polyvinyl butyral 45 Toluene 60 Ethyl acetate 120 i-Propyl alcohol 20 Composition F Composition E was mixed with 0.015 part by weight of cupric oxide.

The characteristics were as follows:

Initial surface potential (volt) 220-250 250-280 Half time for dark attenuation (min.) About 3 About 4 Relative sensitivity for xenon light source 100 Initial potential after pre-exposure to light of 500 lux from a fluorescent lamp for 5 min. and standing in a dark place for 5 min. (volt) -150 In this case, cuprix oxide was reacted as a sensitizer to some extent.

EXAMPLE IV -As to both compositions of Example If to which8 ml. of 0.5% solution of Rose Bengal had been added, a similar effect of promoting the fatigue restoration by the addition of copper salt was found. In this case, the restoration of C was faster with the addition of Rose Bengal.

EXAMPLE v The following two, compositions were applied to an original paper of 80 microns in thickness to be 15 microns:

Composition G Composition L Compound K was mixed with 1 part by weight of cupric stearate.

Each of them was coated on a high-grade paper coated 7 Part by weight 5 with substratum of polyvinyl alcohol in 60 thick to form Photoconductive zinc oxide 100 a dried film of 12 in thickness. The results of measuring Polybutyl methacrylate the preservative stability of the coating compositions Aluminum stearate 0.8 and the characteristics of sensitive layers after drying are Silicone resin varnish (non-volatile 60%) 21 shown in the following table: Sulfonamide resin varnish (non-volatile 50%) 10 10 Ethyl acetate 20 Toluene 20 K L COII'IPOSIIIOH H g gitlzial surfaced potential (volt).-. 320-300 340-380 composition 6 was ix w h 2 parts by wei h f 23% R2100223200530fi ilfiiflaifiii:. 2'58 $53 cupric 1fid (4;l Initial siirlface potegngi)? {after ire-exposure in The characteristics measured: iii iii k z iism e ii 100-120 370-410 (5) Storage stability of liquid composition G H (without cobalt naphthenate) i b a Initial surface potential (volt) .s 300-320 280-300 Half time for dark attenuation (min.) 1 2 Relative sensitivity for xenon light source 100 120 Initial potential after pre-exposure to light of EXAMPLE VIII 1,000 lux from a fluorescent lamp for 5 min. and standing in a dark place for 5min. (volt)- 150-160 240-260 Composition M (control) EXAMPLE VI 25 The following compositions were prepared by mixing Photoconductive Zinc Part by g the followin in redicnts res ectivel g g P y Epoxy-modified alkyd resin (non-volatile 50%) 55 Composition I (control) Part by weight Composition N Photoconductive zinc oxide 100 Silicone resin varnish (non-volatile 60%) 18 Composition M was mixed with 0.7 part by weight Styrene-modified alkyd resin varnish (non-volatile of cupric stearate.

50% 32 Each of compositions M and N was added with 10 Toluene 10 35 parts by weight of a 10% solution of cobalt naphthenate Methyl ethyl ketone 10 in toluene directly before coating and then coated on a composition J steel plate to form a dried film in a thickness of 30-35 Composition I was mixed with 0 4 Part by Weight of Drying was performed by allowing to stand for over night cupric Stearam at room temperature.

Each of the above coating liquid compositions I and J The results are as follows: was coated on a shot-blasted steel plate, respectively, to form a hotoconductive layer in a dried thickness of 15-18 The properties of the layers are shown in the M N f0flowing table, Whefeln terms 2 fi 3 are the results (1) Initialsuriace potential (volt) 450-500 450-500 obtained by measuring after placing in dark chamber for g t f q a k tt u t nf in.) 8 668 more than 10 hours. Further, in the preservative stability (42 tggfgigj; gg gg g gfigg 1 0 of the coating liquid in term 5, symbol a is the case gq gsce z tfiamg 022,020 luxes iofitfi min. 100420 37M) where almost no caking is observed as a result of the fi; ,{g g precipitation of zinc oxide after allowing the coating (Wlthout Cobalt naphthenate) liquid to stand in a closed vessel for more than 2 weeks and formed precipitates were easily redispersed. Symbol b is the case where caking was observed and the redis- What is claimed is:

Pefslofl thereof was dlfiicult- 1. A light sensitive layer for electrophotography which I J consists essentially of hotoconductive zinc oxide powder,

. an electrically insulating film-forming material, and a water insoluble cupric compound in an amount of from a Relative sensitivity to xenon iam ju'fuffi 100 90 0.002% to 20% of the copper ion in said compound by (4) Initial surface potential after pre-exposure in 100-150 420-480 waight of the zinc oxide fluorescent lamp of 500 luxes for 5 min and in dark chamberforSrnin 100-150 42038 2. The light sensitive layer as claimed in claim 1 (5) Preservatwe StabmtY-nb 3 wherein said water-insoluble cupric compound is present As shown in the above table, composition I has diffipredommanfly m the upper 10% to 25% of the surface culties in terms 4 and 5 whereas composition J is excelof the layer lent in terms 4 and 5 as well as in other terms: i layer of damn I Wherem sald cupnc compound 1s cupric stearate.

EXAMPLE VII 4. The layer of claim 2 wherein said cupric compound The followin com unds were re ared: 1S cupnc stearateg p p 5. A method for producing a hotoconductive insulatcomposmon K (control) ing layer comprising an intimate mixture of a photo- Part by weight conductive zinc oxide powder, an organic film-forming Photoconductive i oxide 100 material, and a finely divided insoluble cupric compound, Silicone resin varnish( non-volatile 60%) 35 said n}1Xfl1re provlded a upp Whwh comprlses= Phenol-modified alkyd resin varnish (non-volatile mlxlng a Photoconductlve Zinc oxlde POWder and a 50%) l8 finely divided water-insoluble cupric compound Toluene 20 selected from the group consisting of cupric sulfide,

7 8- cupric selenide, cupric hydroxide, cupric carbonate, References Cited basiccupric carbonate, cupric phosphate, cupric .UNITED STATES PATENTS arsenite, cupric oxalate, cupric oleate, and cupric g stearate with a solution of the organic film-forming 3121O06 2/1964, Middleton et Y 3,197,307 7/1965 Blake et a1. 961.8 materlal and 5 3198632 8/1965 Kimble et a1 96-1 coating the resulting mixture onto the support, where n the mixingratio of thecupric compound to the 3214283 10/1965 Chopoonan,

photoconductive zinc oxide powder is within the NORMAN TORCHIN, Primary Examiner range of from about 0.002z100 to 20:100. a 6. A method of producing aphotoconductive insulat- 10 VAN HORN Asslstant E a ing layer as in claim 5 wherein the film-forming material U 5 Cl X R is a member selected from-the group consisting of alkyd resins and epoxy ester resins. 106296; 252-501 

